This invention relates to a trip circuit for an electric circuit breaker and, more particularly, to a trip circuit that relies upon firing of a thyristor to produce high speed tripping but is still capable of operating to produce tripping even if the thyristor should for some reason fail to fire.
In order to effect high speed tripping of a circuit breaker, it is usually necessary to energize the trip coil of the circuit breaker with a relatively high current. Typically, the protective relay that initiates tripping has light-weight contacts that have limited ability to carry the high tripping current. A conventional way of handling this problem is to provide a seal-in relay that has more rugged contacts than those of the aforesaid protective relay and to operate this seal-in to establish a low impedance by-pass through its rugged contacts around those of the protective relay following initial operation of the protective relay. A problem that is sometimes present in such an approach, however, is that the contacts of the protective relay may have already been damaged by high initial tripping-current before the seal-in relay has had an opportunity to operate and divert current around the protective relay contacts. This problem is an especially troublesome one if the trip circuit is one which is designed to produce a high-amplitude initial pulse of tripping current to accelerate tripping. Such an initial current pulse, if required to traverse the protective relay contacts, would impose severe duty on such contacts.
One way of overcoming this problem is to utilize the initial tripping-current to produce high speed firing of a thyristor in series with the trip coil of the circuit breaker and in parallel with the protective relay contacts, thus diverting the high tripping current around the contacts of the protective relay and through the fired thyristor into the trip coil. My tripping circuit is one which utilizes this approach.